Implantable pulse generator comprising MRI current limiting windings in header structure

ABSTRACT

In one embodiment, a pulse generator for generating electrical stimulation for delivery to a patient, comprises: a hermetically sealed housing containing pulse generating circuitry; a header coupled to the housing for receiving one or more stimulation leads, wherein feedthrough wires are provided to conduct electrical pulses from the pulse generating circuitry to the header; the header comprising a plurality of connectors for electrically connecting to each terminal of the one or more stimulation leads, wherein an inductive winding is disposed around or adjacent to each of the connector structures and is electrically connected between the respective connector structure and a corresponding feedthrough wire to limit MRI induced heating of a respective electrode of the one or more stimulation leads.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/109,866, filed Apr. 25, 2008, now U.S. Pat. No. 8,103,347, whichclaims the benefit of U.S. Provisional Application No. 60/913,908, filedApr. 25, 2007, the disclosure of which is incorporated herein byreference.

BACKGROUND

The present application is generally related to adapting the electricalcontacts within the header of a pulse generator to mitigate or limitcurrent induced in an MRI environment.

Neurostimulation systems are devices that generate electrical pulses anddeliver the pulses to nerve tissue to treat a variety of disorders.Spinal cord stimulation (SCS) is an example of neurostimulation in whichelectrical pulses are delivered to nerve tissue in the spine for thepurpose of chronic pain control. While a precise understanding of theinteraction between the applied electrical energy and the nervous tissueis not fully appreciated, it is known that application of an electricalfield to spinal nervous tissue can effectively mask certain types ofpain transmitted from regions of the body associated with the stimulatednerve tissue. Specifically, applying electrical energy to the spinalcord associated with regions of the body afflicted with chronic pain caninduce “paresthesia” (a subjective sensation of numbness or tingling) inthe afflicted bodily regions. Thereby, paresthesia can effectively maskthe transmission of non-acute pain sensations to the brain.

Neurostimulation systems generally include a pulse generator and one ormore leads. The pulse generator is typically implemented using ametallic housing that encloses circuitry for generating the electricalpulses. The pulse generator is usually implanted within a subcutaneouspocket created under the skin by a physician. The leads are used toconduct the electrical pulses from the implant site of the pulsegenerator to the targeted nerve tissue. The leads typically include alead body of an insulative polymer material with embedded wireconductors extending through the lead body. Electrodes on a distal endof the lead body are coupled to the conductors to deliver the electricalpulses to the nerve tissue.

There are concerns related to the compatibility of neurostimulationsystems with magnetic resonance imaging (MRI). MRI generatescross-sectional images of the human body by using nuclear magneticresonance (NMR). The MRI process begins with positioning the patient ina strong, uniform magnetic field. The uniform magnetic field polarizesthe nuclear magnetic moments of atomic nuclei by forcing their spinsinto one of two possible orientations. Then an appropriately polarizedpulsed RF field, applied at a resonant frequency, forces spintransitions between the two orientations. Energy is imparted into thenuclei during the spin transitions. The imparted energy is radiated fromthe nuclei as the nuclei “relax” to their previous magnetic state. Theradiated energy is received by a receiving coil and processed todetermine the characteristics of the tissue from which the radiatedenergy originated to generate the intra-body images.

Neurostimulation systems are designated as being contraindicated forMRI, because the time-varying magnetic RF field causes the induction ofcurrent which, in turn, can cause significant heating of patient tissuedue to the presence of metal in various system components. The inducedcurrent can be “eddy current” and/or current caused by the “antennaeffect.”

“Eddy current” refers to current caused by the change in magnetic fluxdue to the time-varying RF magnetic field across an area boundingconductive material (i.e., patient tissue). As shown in a simplifiedform in FIG. 1, the time-varying magnetic RF field induces currentwithin the tissue of a patient that flows in closed-paths. Whenconventional pulse generator 103 and conventional implantable lead 104are placed within tissue in which eddy currents are present, implantablelead 104 and pulse generator 103 provide a low impedance path for theflow of current. As depicted in FIG. 1, electrode 102 provides aconductive surface that is adjacent to current path 101. Electrode 102is coupled to pulse generator 103 through a wire conductor (not shown)within implantable lead 104. The hermetically sealed metallic housing(the “can”) of pulse generator 103 also provides a conductive surface inthe tissue in which eddy currents are present. Thus, current can flowfrom the tissue through electrode 102 and out the metallic housing ofpulse generator 103. Because of the low impedance path and therelatively small surface area of electrode 102, the current density inthe patient tissue adjacent to electrode 102 can be relatively high.Accordingly, resistive heating of the tissue adjacent to electrode 102can be high and can cause significant tissue damage.

Also, the “antenna effect” can cause current to be induced which canresult in undesired heating of tissue. Specifically, depending upon thelength of the stimulation lead and its orientation relative to thetime-varying magnetic RF field, the wire conductors of the stimulationlead can each function as an antenna and a resonant standing wave can bedeveloped in each wire. A relatively large potential difference canresult from the standing wave thereby causing relatively high currentdensity and, hence, heating of tissue adjacent to the electrodes of thestimulation lead.

A number of proposals have been published that attempt to mitigate MRIinduced current in a stimulation system. For example, it has beenproposed to couple each wire conductor of a stimulation lead to aninductor. The frequency-dependent characteristic of the inductor tendsto limit the higher frequency MRI currents. The typical approach toimplement the inductor involves wrapping a wire multiple times aroundthe stimulation lead. Additionally, the lead can be specifically adaptedto accommodate the wire windings for the inductor. For example, “bobbin”structures can be placed over the stimulation lead to accommodate thewire windings.

SUMMARY

In one embodiment, a pulse generator for generating electricalstimulation for delivery to a patient, comprises: a hermetically sealedhousing containing pulse generating circuitry; a header coupled to thehousing for receiving one or more stimulation leads, wherein feedthroughwires are provided to conduct electrical pulses from the pulsegenerating circuitry to the header; the header comprising a plurality ofconnectors for electrically connecting to each terminal of the one ormore stimulation leads, wherein an inductive winding is disposed aroundor adjacent to each of the connector structures and is electricallyconnected between the respective connector structure and a correspondingfeedthrough wire to limit MRI induced heating of a respective electrodeof the one or more stimulation leads.

The foregoing has outlined rather broadly certain features and/ortechnical advantages in order that the detailed description that followsmay be better understood. Additional features and/or advantages will bedescribed hereinafter which form the subject of the claims. It should beappreciated by those skilled in the art that the conception and specificembodiment disclosed may be readily utilized as a basis for modifying ordesigning other structures for carrying out the same purposes. It shouldalso be realized by those skilled in the art that such equivalentconstructions do not depart from the spirit and scope of the appendedclaims. The novel features, both as to organization and method ofoperation, together with further objects and advantages will be betterunderstood from the following description when considered in connectionwith the accompanying figures. It is to be expressly understood,however, that each of the figures is provided for the purpose ofillustration and description only and is not intended as a definition ofthe limits of the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts closed-paths of current induced by the time-varyingmagnetic RF field of an MRI system.

FIG. 2A shows a conventional electrical connector.

FIG. 2B depicts another electrical connector in which substantiallyplanar conductive disk is utilized within a metallic band in lieu of aspring.

FIG. 3 depicts assembly for implementing an electrical connector orcontact within a header of a pulse generator according to onerepresentative embodiment.

FIG. 4 depicts an annular conductive structure for use within anelectrical connector according to one representative embodiment.

FIG. 5 depicts a pulse generator with a header containing multipleelectrical connectors according to another representative embodiment.

FIG. 6 depicts a pulse generator header where windings are disposed onspacer structure according to an alternative embodiment.

FIG. 7 depicts a stimulation system according to one representativeembodiment.

FIG. 8 depicts a header adapted for an extension connector that isutilized to electrically couple two leads according to onerepresentative embodiment.

DETAILED DESCRIPTION

Stimulation leads are typically connected to a pulse generator through a“header” that contains various connector structures. The connectorstructures are contained within a suitable housing of epoxy and/orvarious polymers. The connector structures mechanically and electricallytypically couple to the terminals of the stimulation lead(s). Also, theconnector structures are electrically connected to feedthrough wires.The feedthrough wires extend into the hermetically sealed metallichousing that contains the pulse generating circuitry. The term “header”is used in the art, because the header is usually disposed on an uppersurface of the metallic housing (although the header can be disposedanywhere on the device as long as it is reasonably accessible).

FIG. 2A shows a conventional electrical connector, which would belocated in the header of a pulse generator and is used to electricallycouple a terminal of the stimulation lead with a respective feedthroughwire that is, in turn, coupled to the pulse generation circuitry of thepulse generator. As shown in FIG. 2A, connector or contact 200 (which istypically one of several connectors disposed within a header) has acylindrical band having an inner groove bounded by a lower lip and anupper lip. The groove bounded by the upper and lower lip in thisinstance contains a coiled spring, which contacts (electrically couples)to a corresponding terminal of the implantable lead when the lead passesthrough the connector. Such conventional electrical connectors arecommercially available (e.g., from Bal Seal Engineering, Inc. ofFoothill Ranch, Calif.).

Other designs for electrical contacts for use in the header of a pulsegenerator are known. For example, FIG. 2B depicts connector 250 in whicha substantially planar conductive disk is utilized within a band in lieuof the coiled spring which is described in greater detail in U.S. PatentApplication Publication No. 20050107859, entitled “System and method ofestablishing an electrical connection between an implanted lead and anelectrical contact,” which is incorporated herein by reference (nowabandoned). The conductive disk comprises multiple arcuate tabs thatmechanically and electrically couple to the terminals of a lead.

FIG. 3 depicts assembly 400 for implementing an electrical connector orcontact within a header of a pulse generator according to onerepresentative embodiment. Assembly 400 comprises wire 401 which iswound about groove 403 on the outside diameter of ring 410 (shown inisolation in FIG. 4). Any suitable conductive material can be utilizedfor wire 401 such as silver-cored MP35N or stainless steel as examples.An insulative sheath is preferably provided that surrounds theconductive material (except where at the ends of wire 401 whereconnected to other conductive elements). One end of the winding iselectrically coupled to the housing to establish an electricalconnection through the ring 410 (and the associated coiled spring) to aterminal of the stimulation lead. The other end 402 of wire 401 iselectrically coupled to the feedthrough wire (not shown). Thefeedthrough wire is the wire that extends from the header into thehermetically sealed “can” (the metallic housing that contains the pulsegeneration circuitry) and that is coupled to the switch matrix of thepulse generation circuitry. The electrical coupling can be accomplishedby welding the ends of wire 401 to ring 410 and the feedthrough wirerespectively.

The multiple windings of wire 401 about ring 410 provide inductance thatis utilized to suppress MRI induced current. Specifically, theinductance of the winding provides an impedance that limits the currentthat can be passed from the “can” of the pulse generator through thefeedthrough wires to the electrodes of the stimulation lead. The numberof turns of the wire 401 and the material for wire 401 can be selectedto achieve an impedance at stimulation frequencies (e.g., below 5000 Hz)that is relatively low and provides significant attenuation at MRIfrequencies (e.g., 64 MHz).

The inductance of the winding is related the value of μ₀N²A/L, where μis the electrical permeability of free space, N is the number of turnsof the winding, A is the cross section area bounded by the windings, andL is the length of windings. The actual inductance will vary from thecalculated value to some degree due to the inclusion of conductivematerial within ring of the connector upon which windings of wire 401are wrapped. The actual inductance of a winding of wire 401 about aconnector can be readily measured utilizing suitable electronicequipment (e.g., a vector network analyzer). Also, to the extentsufficient attenuation of MRI is achieved, it is preferred to minimizethe length of the winding of wire 401, because the resistance of thewire 401 is related to its length.

There are a number of advantages to disposing wire windings in theheader of a pulse generator rather than within or on the lead itself.One particular advantage is the diameter of the winding is not undulylimited by the diameter of the stimulation lead. Specifically,stimulation leads usually possess relatively minimal outside diametersto facilitate their implantation within a patient. Accordingly, windingsapplied to the lead are somewhat cumbersome to produce. Also, the use of“bobbins” or other structures on stimulation leads to facilitate thewinding process increases the outside diameter and generally prevents anisodiametric profile for the lead. Additionally, the use of the wirewindings in the header of a pulse generator reduces the number ofwindings necessary to achieve the same amount of inductance as would berequired for a smaller diameter. Also, lower stray capacitance isachieved.

FIG. 5 depicts pulse generator 520 with multiple connectors 400 withinheader 500 according to one representative embodiment. Pulse generator520 comprises metallic housing 510 that contains the pulse generatingcircuitry, control circuitry, communication circuitry, battery, etc.Feedthrough wires 503 originate within metallic housing 510 and extendinto header 500 to provide respective electrical connections to thepulse generating circuitry within metallic housing 510. Connectors 400are held in place within respective recesses within the polymerstructure of header 500. Connectors 400 are electrically coupled torespective feedthrough wires 503 to enable electrical connection withrespective terminals of stimulation lead. The inductive windings ofconnectors 400 limit MRI induced current from flowing through connectors400 thereby mitigating MRI induced heating of patient tissue.

In an alternative embodiment as shown in FIG. 6, header 600 of pulsegenerator 650 disposes the windings 401 on spacer structures 601 (whichare electrically non-conductive) that are disposed between conventionalelectrical connectors 20. FIG. 7 depicts stimulation system 700according to one representative embodiment. Stimulation system 700comprises metallic housing 510 coupled to header 500 that is adapted tosuppress or mitigate MRI induced currents by disposing inductivewindings between electrodes 702 of stimulation lead 701 and the metallichousing 510 of the system.

The pulse generation circuitry (not shown) within metallic housing 510can be any suitable pulse generation circuitry now known or laterdeveloped. The pulse generating circuitry can be voltage or currentpulse generating circuitry. Also, the pulse generating circuitry caninclude one pulse source or multiple pulse sources that independentlyproduce output pulses. An examples of pulse generating circuitry may befound in U.S. Patent Application Publication No. 20060170486, entitled“Pulse generator having an efficient fractional voltage converter andmethod of use,” which is incorporated herein by reference (nowabandoned). Also, the pulse generator can be adapted for any particularmedical application such as for spinal cord stimulation, peripheralnerve stimulation, deep brain stimulation, cortical stimulation, cardiacpacing or defibrillation, etc. Lead 701 can be any suitable stimulationlead that is capable of being received within a header of a stimulationsystem either now existing or later developed. Lead 701 can be apercutaneous lead or a paddle-style lead. Suitable commerciallyavailable leads include Axxess® and Lamitrode® leads available fromAdvanced Neuromodulation Systems, Inc. of Plano, Tex.

In another alternative embodiment, header 500 can be disposed withinlead extension connector 801 that is utilized to electrically couple twoleads 701 as shown in FIG. 8. Extension connector 801 includes animplantable housing that has a first and second aperture for receivingelectrodes of a first stimulation lead and for receiving terminals of asecond stimulation lead. A first plurality of electrical connectors aredisposed within the first aperture for electrically connecting to theelectrodes of the first stimulation lead. A second plurality ofelectrical connectors are disposed within the first aperture forelectrically connecting to the electrodes of the first stimulation lead.Also, a plurality of connector wires are provided for electricallycoupling the first plurality of electrical connectors with the secondplurality of electrical connectors. MRI induced current is suppressed ormitigated by utilizing windings according to representative embodiments.For example, at least one of the first and second plurality ofelectrical connectors comprises a respective inductive winding disposedaround or adjacent to each of the electrical connectors. Each inductivewinding is electrically disposed between the respective electricalconnector and a corresponding connector wire to limit MRI inducedcurrent. The inductive windings are preferably welded between respectiveconnectors wires and electrical connectors.

Although representative embodiments and advantages have been describedin detail, it should be understood that various changes, substitutionsand alterations can be made herein without departing from the spirit andscope of the appended claims. Moreover, the scope of the presentapplication is not intended to be limited to the particular embodimentsof the process, machine, manufacture, composition of matter, means,methods and steps described in the specification. As one of ordinaryskill in the art will readily appreciate from the disclosure thatprocesses, machines, manufacture, compositions of matter, means,methods, or steps, presently existing or later to be developed thatperform substantially the same function or achieve substantially thesame result as the corresponding embodiments described herein may beutilized. Accordingly, the appended claims are intended to includewithin their scope such processes, machines, manufacture, compositionsof matter, means, methods, or steps.

1. A method of providing cortical or deep brain stimulation to a patientusing an implantable pulse generator and a stimulation lead, the methodcomprising: providing a connector implant device, the connector implantdevice comprising (i) an implantable housing, (ii) an aperture withinthe housing for receiving a portion of the stimulation lead, (iii) aplurality of electrical connectors within the aperture for electricallyconnecting to the electrical contacts of the stimulation lead, whereineach of the plurality of electrical connectors comprises a respectiveouter band, and (iv) a respective inductive winding disposed around theouter band of each of the electrical connectors, wherein the respectiveinductive winding is adapted to limit MRI induced current flowing to orfrom the stimulation lead; inserting the stimulation lead into theconnector implant device; connecting the implantable pulse generator tothe stimulation lead through the connector implant device; generatingstimulation pulses by the implantable pulse tissue; and delivering thegenerated stimulation pulses to cortical or deep brain tissue of thepatient using the stimulation lead.
 2. The method of claim 1 whereineach outer band of the plurality of electrical connectors comprises agroove on an outside diameter of the band to accommodate itscorresponding inductive winding.
 3. The method of claim 2 wherein eachelectrical connector comprises a coiled spring within an interior of theouter band, wherein the coiled spring is adapted to electrically engagean electrical contact of the stimulation lead.
 4. The method of claim 1wherein the stimulation lead is a paddle lead adapted to provideelectrical stimulation to cortical tissue of the patient.
 5. The methodof claim 1 wherein the stimulation lead comprises a substantially roundcross-sectional profile.
 6. The method of claim 1 wherein each inductivewinding is welded to the outer band of its corresponding electricalconnector.
 7. The method of claim 1 wherein each inductive winding isadapted to substantially limit current near 64 MHz.
 8. The method ofclaim 1 wherein each inductive winding is adapted to substantially passcurrent under 5000 Hz.